The present invention generally relates to high definition television (HDTV) systems and, particularly concerns an improved non-compatible HDTV system. As used herein, an HDTV system refers to a system designed for transmitting a television signal resulting in a displayed image at a receiver having a substantially improved resolution relative to the current NTSC system.
Many HDTV systems are currently under study, all having the purpose of improving the resolution afforded by the current NTSC standard system. In the NTSC system, a television signal is transmitted in the form of a plurality of video lines, the lines being arranged in successive fields for transmission. The lines of adjacent fields are interlaced with each other to provide a complete frame of video information in two fields. The lines in each field are transmitted at a horizontal rate of 15.74 KHz and the fields at a rate of 59.94 Hz. The horizontal resolution of the video signal is limited to about 4 MHz for transmission through a standard 6 MHz RF television channel.
The proposed HDTV systems may generally be categorized into three groups; compatible systems, augmentation systems and non-compatible systems. Compatible systems add to or modify an existing NTSC transmission in a manner so that it may still be viewed on an existing receiver with NTSC resolution, but may be received with higher resolution on a special HDTV receiver. The compatible transmission is normally limited to the same 6 MHz channel used for NTSC transmissions. Augmentation systems, on the other hand, transmit augmenting signals in additional spectrum space without disturbing the NTSC transmission. Thus, an existing receiver may reproduce the televised image with NTSC resolution by tuning the standard channel, while a special HDTV receiver may be used to tune both the standard channel and the augmenting signals to reproduce an HDTV image. Transmissions comprising various combinations of these two techniques are also possible.
Non-compatible systems provide HDTV transmissions which cannot be received by standard NTSC receivers. Since such transmissions must be made over unused spectrum, care must be exercised to avoid interference with existing NTSC channels. Cochannel and adjacent channel interference is of particular concern in this regard. The above referenced copending application discloses techniques for minimizing such interference. These techniques may be used with non-compatible, as well as augmentation-type, HDTV systems to allow maximum utilization of existing spectrum.
It is believed that a non-compatible system will provide the best solution to the problem of providing high quality HDTV. Compatible systems have the disadvantage of almost inevitably producing undesired artifacts in standard NTS receivers. Augmentation systems have the disadvantage that multiple RF bands must be tuned and that different signals must be precisely pieced together to form the HDTV image.